Final answer:
X-linked agammaglobulinemia is caused by a defect in the BTK gene, resulting in a halt in B-cell maturation and a lack of specific antibody production.
Step-by-step explanation:
X-linked agammaglobulinemia is caused by a defect in the BTK gene, which encodes a tyrosine kinase called Bruton tyrosine kinase (Btk). This defect leads to a halt in B-cell maturation and differentiation at the pre-B-cell stage. As a result, immunoglobulin production is impaired, leading to a lack of specific antibody production.
Individuals with X-linked agammaglobulinemia have a higher susceptibility to recurrent infections caused by extracellular pathogens. This is because their immune system is unable to produce sufficient antibodies to fight off these pathogens.
While their cell-mediated immunity is not impaired, meaning they are not particularly vulnerable to infections caused by viruses or intracellular pathogens, they are at a higher risk of pyogenic infections caused by bacteria such as Haemophilus influenzae, Streptococcus pneumoniae, S. pyogenes, and S. aureus.